In the cost breakdown of light emitting diode (LED) luminaires, the component to circuitry interconnect solution, or when the component is part of the circuitry, herein under referred to as a level two (L2) interconnect, is becoming increasingly important because of two main reasons. Firstly, the LED costs are decreasing, and secondly, in many LED luminaire designs, there is little room left to have a cost down on for example the housing parts. Both reasons lead to a relative increase of importance of the L2 interconnect on the total system costs.
FIG. 1 schematically illustrates a typical L2 interconnect in which a component, here a packaged LED 10, is interconnected with a LED board 50 being a printed circuit board, PCB, by means of soldering. A LED board is usually provided as a stack. The LED board 50 comprises a carrying substrate 51 for providing a required robustness, or flexibility, of the LED board 50. One or more dielectric layers 55 for providing a basic insulation of the LED board 50 is typically laminated together with epoxy resin onto the substrate 51. On top of the substrate 51, a conductor layer is also laminated over the full surface area. The conductor layer is thereafter chemically etched to provide for the final conductor structure 52 and circuitry. This etching process is by nature a discontinuous batch process. The LED 10 is interconnected to the conductor structure 52 by means of soldering. Before applying solder 53 to the LED board it is typically coated with a solder mask, a patterned solder resist layer 54, defining areas where solder is to be applied. The solder resist layer 54, which is typically 20-30 micrometers thick, may be a polymer coating applied in an offset process of a dispensed and cured polymer material. The solder resist layer 54 prevents solder from bridging between conductors 52 thereby creating short circuits, and may further provide protection from the environment.
Whereas the whole stack of a PCB is typically produced by lamination, the actual final end result for the conductor circuitry 52 and solder resist 54 is created by a discontinuous process. These batch processes do not come down in cost significantly with larger volumes in production.
Further, with respect to material utilization, there is little flexibility in playing with the essential and valuable conductor layer properties. When providing the conductor structure 52, firstly a copper layer is applied to the full L2 process plate surface, followed by patterning and removing of copper, which cost time and saturates the chemical etching dissolvent. Growing a thicker layer, for better heat management, also cost extra time and energy.
In addition to the standard PCB type of L2 interconnect described above, there are many other types of L2 interconnects, which are mostly not relevant for reasons of high cost and complexity. One solution that is of relative low cost and which may be used for less complex circuitries is using lead-frames. In general this means putting components on a rigid, possibly bended, conductor frame, which is processed in a final stage to provide for the required circuitry. The lead-frame can be produced in different ways depending mostly on size and complexity e.g. mechanical stamping or chemical etching. There are some typical drawbacks to this approach. Firstly with creating the final circuitry the initial lead-frame will lose its mechanical integrity literally falling apart. One can either design to have mechanical stresses going through the electrical components such as is typical in larger mechanical lead-frames, or one can introduce some feature, e.g. plastic overmoulding, to provide for the necessary rigidity while the final circuitry is created before electrical components can be placed. Furthermore in general these types of solutions do not provide for the necessary electronic insulation requirements whereas dielectrics are not or only applied in limited areas. Prescribed creepage and clearance distances are difficult to manage or incorporate into the L2 interconnect design and must mostly be managed on a luminaire/system level. Finally if one wants to optimize for thermal management and the heat spreader and/or heat sink is made out of conductive material one has to introduce a separate dielectric component on a luminaire/system level.